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I have a joint between two sheets of cement board that forms a small ridge elevated maybe 1/8" relative to the centers of either board. I was wondering if extra thinset on both sides can be used to even it out or is it better to use self-leveling compound beforehand. I am using expensive marble tiles and don't want them to break to an unevenness.

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up vote 3 down vote accepted

You may break a few pieces(of marble) just picking them up.

A skim coat on either side of the joint will help.

Block feather with a concrete 'rubbing' block.

My marble floor always have Schluter Ditra under them. I also use it with any questionable floor.

Ceramic tile can tolerate L/360 deflection, stone is twice as sensitive at L/720.

If you have any doubts about floor springiness, use an isolation membrane or stiffen joists or add a beam. Cement board does little for stiffness.

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a skim coat of what? –  amphibient Apr 8 '13 at 20:59
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Thinset mortar. –  HerrBag Apr 8 '13 at 21:32
    
what if i just dump some self-leveling on either side and wait for it to solidify before setting thinset? –  amphibient Apr 8 '13 at 21:35
    
as far as Schluter Ditra, do i set that using thinset and then again thinset on top between it and the tile? –  amphibient Apr 8 '13 at 21:44
    
You can use leveling mortar first. Ditra is set based on the substrate, usually polymer modified thinset over ply or cement board. For most marble, a white thinset is best. –  HerrBag Apr 8 '13 at 22:59
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